AIDS, Acquired Immune Deficiency Syndrome, is a catastrophic disease that has reached global proportions. Currently an estimated 40 million people worldwide are living with AIDS, with approximately 5 million new infections every year. The yearly death toll is still over 3 million people worldwide. Another virus that causes a serious human health problem is the hepatitis B virus (HBV). HBV is second only to tobacco as a cause of human cancer. Some estimates put the number of people worldwide that have been infected with HBV as high as two billion people, up to a third of the world's population, with approximately 400 million chronically infected.
A number of 2′,3′-dideoxynucleosides have been found to be potent antiviral agents against HIV and/or hepatitis B virus. After cellular phosphorylation to the 5′-triphosphate by cellular kinase, these synthetic nucleosides are incorporated into a growing strand of viral DNA, causing chain termination due to the absence of the 3′-hydroxyl group. They can also inhibit the viral enzyme reverse transcriptase.
There has also been interest in the synthesis of nucleoside derivatives in which the 3′-carbon of the nucleoside has been replaced with a heteroatom. Both 3TC and its 5-fluorocytosine analog (FTC) exhibit activity against HIV and HBV (Furman, et al., Antimic. Ag. Chemo., 1992, 2686-2692; and Cheng, et al., J. Biol. Chem., 1992, 267(20), 13938-13942).
The discovery that a racemic oxathiolane nucleoside BCH-189 possessed a potent activity against replication of HIV prompted Chu et al. to synthesize the chiral products (+)- and (−)-BCH-189 (Belleau, et al., 5th International Conference on AIDS, Montreal, Canada, Jun. 4-9, 1989, #T.C.O. 1; Chu, et al. Tetr. Lett., 1991, 32, 3791). The latter compound, lamivudine, otherwise known as 3TC or epivir, is currently used clinically in the treatment of both HIV infection and HBV infection. The (−) enantiomer of 5-fluorocytosine oxathiolane analogue (FTC), is particularly active against HIV (Choi, W. et al., J. Am. Chem. Soc., 1991, 113, 9377; Schinazi, R. F., et al., Antimic. Ag. Chemo. 1992, 2423; U.S. Pat. Nos. 5,204,4665; 5,210,085; 5,914,331; and 5,639,814).
The above-described 1,3-oxathiolane nucleosides are manufactured by condensation of silylated purine or pyrimidine base with a 1,3-oxathiolane intermediate. U.S. Pat. No. 5,204,466 discloses a method to condense a 1,3-oxathiolane with a silylated pyrimidine using tin chloride as a Lewis acid, which provides virtually complete β-stereoselectivity. A number of U.S. patents describe processes for the preparation of 1,3-oxathiolane nucleosides via condensation of a 1,3-oxathiolane-2-carboxylic acid ester with a protected silylated base in the presence of a silicon-based Lewis acid, followed by reduction of the ester to the corresponding hydroxymethyl group to afford the final product (see U.S. Pat. Nos. 5,663,320; 5,693,787; 5,696,254; 5,744,596; 5,756,706 and 5,864,164). In addition, these patents contain generic disclosures for the synthesis of 1,3-dioxolane nucleosides in a similar fashion using the corresponding 1,3-dioxolane intermediate.
U.S. Pat. No. 5,272,151 discloses a process using a 2-O-protected-5-O-acylated-1,3-oxathiolane for the preparation of nucleosides by condensation with a silylated purine or pyrimidine base in the presence of a titanium catalyst. U.S. Pat. No. 6,215,004 discloses a process for producing 1,3-oxathiolane nucleosides that includes condensing 2-O-protected-methyl-5-chloro-1,3-oxathiolane with a silylated 5-fluorocytosine without a Lewis acid catalyst. In these cases, the 1,3-oxathiolane ring is prepared in one of the following ways: (i) reaction of an aldehyde derived from a glyoxylate or glycolic acid with mercaptoacetic acid in toluene in the presence of p-toluenesulfonic acid to give 5-oxo-1,3-oxathiolane-2-carboxylic acid; (ii) cyclization of anhydrous glyoxylates with 2-mercaptoacetaldehyde diethylacetal at reflux in toluene to give 5-ethoxy-1,3-oxathiolane lactone; (iii) condensation of glyoxylic acid ester with mercaptoacetaldehyde (dimeric form) to give 5-hydroxy-1,3-oxathiolane-2-carboxylic ester or (iv) coupling of an acyloxyacetaldehyde with 2,5-dihydroxy-1,4-dithiane, the dimeric form of 2-mercaptoacetaldehyde to form a 2-(ayloxy)methyl-5-hydroxy-1,3-oxathiolane. The lactone, 5-oxo compound, has to be reduced to the corresponding lactol during the process. The 2-carboxylic acid or its ester also has to be reduced to the corresponding 2-hydroxymethyl derivatives with borane-methylsulfide complex.
The key intermediate, aldehyde, can be prepared using several methods: (i) lead tetraacetate oxidation of 1,4-di-O-benzoyl meso-erythritol, 1,6-di-O-benzoyl D-mannitol or 1,5-di-O-benzoyl-D-arabitol; (ii) preparation of monoacylated ethylene glycol followed by oxidation to aldehyde; (iii) acylation of ethylene chlorohydrin followed by dimethylsulfoxide oxidation; (v) lead tetraacetate oxidation; (vi) ozonolysis of allyl or 3-methyl-2-buten-1-ol acylate; (vii) and more recently, by acylation of 2-butene-1,4-diol followed by ozonolysis. Also, U.S. Pat. No. 6,215,004 discloses a process to prepare acyloxyacetaldehyde diethylacetal by acylation of 2,2-diethoxyethanol.
Norbeck, D. W., et al. (Tet. Lett., 1989, 30, 6263) reported the synthesis of (±)-1-(4β,4β)-2-(hydroxymethyl)-4-dioxolanyl-thymine, that results in a racemic mixture of diastereomers about the C4′ atom. The X-ray crystallographic analysis of the product revealed that the dioxolane ring adopts the 3T4 conformation commonly observed in ribonucleosides, with the O3′ atom in the endo position, which is quite distinct from the distorted 3E conformations observed in AZT, AZDU, ddA, ddC, and 3′-deoxy-3′-fluorothymidine, all of which exhibit potent in vitro activity against HIV.
The antiviral activity of dioxolane nucleosides prompted Chu et al. to synthesize a series of analogs in a search for potent antiviral and/or anticancer agents. For example, 9-(β-D hydroxymethyl-1,3-dioxolanyl)aminopurine (β-D-DAPD), and its metabolite 9-(β-D hydroxymethyl-1,3-dioxolanyl)-guanine (β-D-DXG) have been reported to have potent and selective activity against human immunodeficiency virus (HIV) and hepatitis B virus (HBV) (Rajagopalan et al., Antiviral Chem. Chemother., 1996, 7(2), 65-70). (−)-DAPD is a potent and selective inhibitor of HIV in vitro and in vivo and HBV replication in vitro (Furman, et al. Drugs of the Future 2000, 25 (5), 454-461). Similarly, 1-(β-L hydroxymethyl-1,3-dioxolanyl)-thymine (Dioxolane-T) (Norbeck et al., Tet. Let., 1989, 30, 6263-66) possess anti-HIV and anti-HBV activity. 1-(β-L hydroxymethyl 3-dioxolanyl)-cytidine (β-L-OddC) was discovered to have potent anti-tumor activity towards human prostate as well as renal carcinoma (Kadhim et al., Can. Cancer Res., 57(21), 4803-10, 1997). (−)-(2′S,4′R)-1′-[2′-(hydroxy-methyl)-1′,3′-dioxolan-4′-yl]-5-iodouracil) L-IOddU is currently in pre-clinical or clinical studies to assess its value as an antiviral or anticancer agent (see Kim, et al., J. Med. Chem. 1993, 36, 519-528 and references therein; Corbett, & Rublein, Curr. Opin. Investig. Drugs 2001, 2, 348-353; Gu, et al., Antimicrob. Agents Chemother. 1999, 43, 2376-2382; Mewshaw, et al., J. Acquir. Immune Defic. Syndr. 2002, 29, 11-20).
U.S. Pat. No. 5,041,449 and U.S. Pat. No. 5,270,315 to Belleau et al. disclose a generic group of racemic 2-substituted-4-substituted-1,3-dioxolanes. Table 1 of the reference shows data for two racemic 1,3-dioxolane nucleosides—a racemic trans (α) 1,3-dioxolane nucleoside with a cytosine base (Compound XII) and a racemic cis (β) 1,3-dioxolane nucleoside with an adenine base (Compound XIV) (see also EP 0 337 713 to IAF BioChem International).
In June 1989, Belleau, et al., reported a method of synthesis of cytidine nucleosides that contain oxygen or sulfur in the 3′-position (Belleau, B., et al. Fifth International Conference on AIDS, Montreal; International Development Research Centre: Ottawa, Ontario, 1989; T.C.O.1.). The dioxolane ring was prepared by the condensation of RCO2CH2CHO with glycerin. The synthesis resulted in a racemic mixture of diastereoisomers about the C4′ carbon of the nucleoside. Racemic DAPD was synthesized as depicted in Scheme 1.

Belleau et al. reacted glycerol and chloroacetaldehyde to generate a dioxolane intermediate. After chlorine displacement with a benzoic acid salt, oxidation of the primary alcohol to a carboxylic acid and Baeyer-Villiger rearrangement with m-chloroperbenzoic acid, the corresponding racemic dioxolane benzoate was obtained. This compound was then coupled with 2-amino-6-chloropurine and the resulting nucleoside intermediate was reacted with ammonia under pressure to afford racemic DAPD. (±)-Dioxolane-T was also synthesized in similar fashion by Choi et al. (Choi, et al., J. Am. Chem. Soc. 1991, 113, 9377-9378 and U.S. Pat. No. 5,852,027).
As discussed above, in late 1989, Norbeck et al. published an article which described the synthesis of racemic cis-1,3-dioxolane thymidine which also had anti-HIV activity in vitro (Norbeck, et al. Tet. Let. 1989, 30 (46), 6263-6266). The product was synthesized in five steps from benzyloxyaldehyde dimethylacetal and (±)-methyl glycerate to produce a 79% yield of the 1:1 diastereomeric mixture. As with the Belleau synthesis, the Norbeck synthesis results in a racemic mixture of diastereoisomers about the C4′ carbon of the nucleoside. See Scheme 2.

The same racemic dioxolane acetate intermediate was synthesized by Liotta et. al. starting from cis-2-buten-1,4-diol (Choi, et al. J. Am. Chem. Soc. 1991, 113 (24), 9377-0379 and Wilson, et al. Bioorg. Med. Chem. Let. 1993, 3 (2), 169-174). See Scheme 3.

The drawback of these procedures, shown in Scheme 3, is that they involve the synthesis of an unstable aldehyde and difficult oxidative step(s).
U.S. Pat. No. 5,179,104 to Chu and Schinazi, discloses a method to obtain enantiomerically pure β-D-1,3-dioxolane nucleosides via a stereospecific synthesis (see also related U.S. Pat. Nos. 5,925,643; 5,767,122; 5,444,063; 5,684,010; 5,834,474; and 5,830,898).
EP 0 515 156 to BioChem Pharma discloses a method to obtain the enantiomers of 1,3-dioxolane nucleosides using a stereoselective synthesis that includes condensing a 1,3-dioxolane intermediate covalently bound to a chiral auxiliary with a silyl Lewis acid (see also related U.S. Pat. Nos. 5,753,706 and 5,744,596).
Chu, et al., published a stereospecific synthesis of β-D-1,3-dioxolane nucleosides from 1,6-anhydromannose (Chu, et al. Tet. Let., 1991, 32, 3791-3794). At about the same time, Thomas and Surber published an article which described that (i) a thorough search of the literature on chiral chromatography failed to reveal any examples of nucleoside separations and that (ii) their paper appears to be the first separation of the enantiomers of a nucleoside by chiral high performance liquid chromatography. The nucleoside resolved was not a 1,3-dioxolane nucleoside, and four out of the five chiral columns attempted did not work (Thomas, et al., J Chromat., 1991, 586, 265-270).
Kim et al. (Kim, et al. J. Med. Chem. 1993, 36 (1), 30-37) subsequently published a paper which discloses an asymmetric synthesis of 3-D and α-D enantiomers of 1,3-dioxolane pyrimidine nucleosides from 1,6-anhydro-D-mannose. The synthesis of (−)-DAPD was described as a thirteen step process from 1,6-anhydro-D-mannose including a nine step conversion of 1,6-anhydro-D-mannose to a chiral acetate (Scheme 4).

After coupling of the acetate under Vorbruggen conditions and several purification and deprotection steps, (−)-DAPD was obtained in modest yield. This process is time consuming, difficult and involves complicated oxidation steps.
In 1992, Belleau et al. (Belleau, et al. Tet. Let. 1992, 33, 6949-6952) published a synthesis of enantiomerically pure 2′,3′-dideoxy-3-oxacytidine stereoisomers via an eight step process using L-ascorbic acid as a chiral auxiliary. L-ascorbic acid was used to produce a set of diastereomers which could be separated. The use of lead tetraacetate makes this process unsuitable for scale-up. In 1992, Kim, et al., also published an article disclosing how to obtain (−)-L-β-dioxolane-C and (+)-L-β-dioxolane-T from 1,6-anhydro-L-β-gulopyranose (Kim, et al., Tet. Let. 1992, 32(46), 5899-6902).
Liotta and Shinazi, in U.S. Pat. No. 5,276,151, found that racemic 2-O-protected-5-O-acylated-1,3-dioxolanes could be coupled with purine or pyrimidine bases in the presence of a titanium-containing Lewis acid to predominately generate the racemic (3-isomers (see also WO 92/14729).
Jin et al. (Jin, et al. Tet. Asym. 1993, 4 (2), 211-214), discloses that Lewis acid catalysts play a crucial role in the preparation 1,3-dioxolane nucleosides. TiCl4 and SnCl4 promote the formation of dioxolane nucleosides with racemization in the coupling of enantiomerically pure 2′-deoxy-3′-oxaribosides with silylated N-acetylcytosine. The use of the Lewis acids trimethylsilyltriflate, trimethylsilyl iodide, and TiCl2(Oi-Pr)2 furnishes enantiomerically pure cytosine dioxolane nucleosides in low diastereoselectivity.
An asymmetric synthesis of dioxolane nucleosides was reported by Evans, et al. (Tet. Asym. 1993, 4, 2319-2322). Reaction of D-mannitol with BnOCH2CH—(OCH3)2 in the presence of SnCl2 in 1,2-dimethoxyethane followed by RuCl3/NaOCl oxidation gave cis- and trans-dioxolane-4-carboxylic acid, which was then converted to D- and L-dioxolane nucleosides by decarboxylation, coupling and deprotection reactions. An alternative route to these carboxylic acids by reaction of BnOCH2CH(OCH3)2 with L-ascorbic acid was also reported in the paper. The chiral carboxylic acid can also be prepared by reacting commercially available 2,2-dimethyl-1,3-dioxolane-4-(S)-carboxylic acid with a protected derivative of hydroxy-acetaldehyde such as benzoyloxyacetaldehyde, under acidic conditions (see U.S. Pat. Nos. 5,922,867 and 6,358,963).
Siddiqui, et al., discloses that cis-2,6-diaminopurine dioxolane can be selectively deaminated using adenosine deaminase (Siddiqui, et al., Bioorg. Med. Chem. Let., 1993, 3 (8), 1543-1546).
While the synthesis of dioxolane nucleosides is possible using processes described in the literature, the chemistry is not applicable to the synthesis of (−)-DAPD on large scale. See Chu, et al. Tet. Let. 1991, 32, 3791-3794; Siddiqui, et al. Bioorg. Med. Chem. Let. 1993, 3, 1543-1546; Kim, et al. Tet. Let. 1992, 46, 6899-6902).
U.S. Pat. No. 5,763,606 to Mansour et al. describes processes for producing predominately pure 1,3-oxathiolane and 1,3-dioxolane nucleosides via coupling of a silylated purine or pyrimidine base with a bicyclic intermediate (see also WO 94/29301).
U.S. Pat. No. 6,215,004 to Painter et al. discloses process for preparing 2-[R1C(O)OCH2]-1,3-dioxolanyl-5-one by reacting glycolic acid with an acetal of the formula (R1O)2CHR; a hemiacetal of the formula (R2O)(HO)CHR; or a mixture thereof, wherein R is —(CH2—O—C(O)R1), and R1 and R2 are independently alkyl, aryl, heteroaryl, heterocyclic, alkaryl, alkylheteroaryl, or alkylheterocyclic, or aralkyl, in the presence of a Lewis acid, such as boron trifluoride diethyl etherate (see also WO 00/09494).
WO 00/47759 and WO 01/58894 both to BioChem Pharma disclose processes of separating β and α anomers from an anomeric mixture of dioxolane analogs with a COOR moiety at the C4′ position prior to coupling with a purine or pyrimidine base. The process for resolving the dioxolane analogues to obtain dioxolanes having a predominant β-L-configuration, involves the use of enzymes, namely hydrolases.
WO 03/062229 to Shire BioChem. Inc. discloses a single reaction vessel process for producing a dioxolane nucleoside analogue by adding a Lewis acid, a silylating agent and a non-silylated purine or pyrimidine base to a dioxolane. The publication also describes a process for producing a dioxolane compound by reacting a dioxolane compound in a solvent in the presence of DIB and I2, using a suitable source of energy.
The stereochemistry of 3′-oxa-substituted 2′,3′-dideoxynucleoside analogues (“dioxolane nucleoside analogues”) can play an important role in their biological activity. The Cr position of the ribose in the nucleoside is a chiral center because the carbon is attached to four different moieties. Likewise, there is an optically active center at C4′ of the nucleoside.

As shown below, the substituents on the chiral carbons (the specified purine or pyrimidine base and CH2OH) of 1,3-dioxolane nucleosides can be either cis (on the same side) or trans (on opposite sides) with respect to the dioxolane ring system. For the purpose of consistency, the same stereochemical designation is used when the methyloxy moiety or the base moiety is replaced with another substituent group. Both the cis and trans racemates consist of a pair of optical isomers. Hence, each compound has four individual optical isomers. The four optical isomers are represented by the following configurations (when orienting the dioxolane moiety in a horizontal plane such that the —O—CH2— moiety is in front): (1) cis, with both groups “up”, which is a β-cis configuration (referred to as β-D); (2) cis, with both groups “down”, which is the opposite β-cis configuration (referred to as β-L); (3) trans with the C4′ substituent “up” and the Cr substituent “down”; and (4) trans with the C4′ substituent “down” and the Cr substituent “up”. The two cis enantiomers together are referred to as a racemic mixture of β-enantiomers, and the two trans enantiomers are referred to as a racemic mixture of α-enantiomers. In general, it is difficult to separate or otherwise obtain the individual enantiomers of the cis-configuration. The four possible stereoisomers of cis-1,3-dioxolane nucleosides are illustrated below:

Since stereoisomers of dioxolane nucleosides usually have different biological activities and toxicity, obtaining the pure therapeutically active isomer becomes crucial. Frequently, one stereoisomer is considerably more active than the other.
Chu et al. developed methods for the asymmetric synthesis of dioxolane nucleosides from D-mannose and L-gulonic lactone for D- and L-dioxolane nucleosides, respectively (U.S. Pat. Nos. 5,767,122, 5,792,773). However, these processes involved many steps and most of the intermediates need to be purified by silica gel column chromatography (see Kim et al. J. Med. Chem. 1993, 36, 519-528).
To prepare a sufficiently large quantity of dioxolane nucleoside drug substance for clinic trials, a chiral 2-acyloxymethyl-5-oxo-1,3-dioxolane has been used as the key intermediate. This was prepared by cyclization of ROCH2CHO or its acetal with glycolic acid in the presence of BF3, followed by column separation on chiral resin or by enzymatic resolution, which are expensive and difficult techniques.
Thus, there remains a need for cost-effective and stereoselective processes to produce biologically active isomers of dioxolane nucleosides.
It is an object of the present invention to provide novel and cost-effective processes for the synthesis of enantiomerically pure dioxolane nucleosides.